1. Field of the Invention
The present invention relates to a power supply apparatus which regulates an input voltage from a power source such as a battery so as to generate a predetermined output voltage and which protects against an overcurrent.
2. Description of the Related Art
A power supply apparatus of a series type, capable of generating a predetermined output voltage fed to a load and limiting a load current to a predetermined value, is known in the related art (patent document No. 1).
Referring to FIG. 1 of the patent document No. 1, the power supply apparatus of a series type is configured such that a control transistor P1 is provided between an input terminal and an output terminal. The control transistor P1 is controlled by an operational amplifier OP1 operated on the basis of an output voltage Vo and a reference voltage Vref1 so that the output voltage Vo maintained to be constant is supplied to a load Z1. There is also provided a current detection transistor P2, which is controlled concurrently with the control transistor P1 by the same control signal as the that of the control transistor P1. With this arrangement, a current indicator value Vb proportional to an output current is detected. A comparator COMP1 compares the current indicator value Vb with a predetermined current limit value Vref2. When the current indicator value exceeds the current limit value, a comparison output is generated so that the control transistor P1 is turned off.
[Patent document No. 1 ]
JP2003-173211
With the related-art power supply apparatus of patent document No. 1, a predetermined output voltage generated from an input voltage is supplied to a load. An excessive output current is limited to a predetermined value even when a failure such as a short circuit at the load occurs.
The power supply apparatus of patent document No. 1 generates a comparison output when the current indicator value exceeds the current limit value so as to turn the control transistor off. As such, the related-art apparatus is incapable of supplying an output current in excess of a level commensurate with the current limit value to the load, even when the output voltage is permitted to be lower than the predetermined voltage, or when the load is heavy, for example. Another disadvantage is that, since the gain in an overcurrent limiting operation is high, a voltage drop in a output voltage vs. output current plot (drop characteristic) is steep. This results in an increased possibility of oscillation occurring in the current limiting operation.
Further, the same control signal as used to control the control transistor is used to control the current detection transistor for simultaneous control in the power supply apparatus of patent document No. 1. Therefore, the overcurrent limiting operation cannot be controlled independent of an operation for controlling the control transistor.